The present invention generally relates to an image signal average picture level detecting apparatus which may be used in the detection of the average picture level (hereinafter referred to as APL) of image signals, such detection being necessary in a case where, for example, gradation correcting of video luminance signals is effected in image appliances such as television image receivers, video tape recorders and so on.
As color television image receivers have become larger in recent years, various gradation correcting apparatuses have been used to correct the gradation of image signals for the purpose of displaying the images more clearly and expanding the dynamic range of the images on the CRT. In order for the gradation correcting apparatus to effect the gradation correcting operation, it is necessary to detect the average picture level of the image signals. Therefore, the performance of the image signal average picture level detecting apparatuses has become more important.
The conventional image signal average picture level detecting apparatus will be described hereinafter with reference to FIGS. 4-6.
FIG. 4 shows a block diagram of the conventional image signal average picture level detecting apparatus. In FIG. 4, reference numeral 1 is a pedestal clamping circuit for clamping the pedestal of the input video luminance signal "a" to a pedestal voltage "b" (FIG. 5a). Reference numeral 2 is a synchronous separating circuit for separating a synchronous signal "e" from the input video luminance signal "a". Reference numeral 3 is a video blanking signal generating circuit for waveform shaping the synchronous signal "e" so as to generate a video blanking signal "f" (FIG. 5b). Reference characters Tr3 and RO constitute a constant current supply 4 for generating a current I (FIG. 5c) when the video blanking signal "f" is in an image period. The current I is not generated when the video blanking signal "f" is in an image blanking period. The constant power supplies Tr1, Tr2, Ri, R8 constitute a differential amplifying circuit 5 for comparing the pedestal voltage "b" with a post-clamp video luminance signal "c" so as to generate currents I1 (FIG. 5c) and I2 (FIG. 5d) into the collectors of the respective transistors. As the amplitude of the video luminance signal becomes larger, the current I2 increases, and the current I1 decreases proportionately. The constant power supplies Tr7, Tr8, R6, R7 constitute a current mirror circuit 9, and the current I5 (FIG. 5d) equal to the current I2 which changes in accordance with the amplitude of the video signal is generated into the collector of Tr8. The constant power supplies R.sub.L, C.sub.L and V.sub.P convert the current I5 into a voltage and average them so as to constitute a smoothing circuit 8 for generating an average picture level "d" (FIG. 5f).
The operation of the conventional image signal average picture level detecting apparatus will be described hereinafter.
The input video luminance signal "a" is inputted into a synchronous separating circuit 2 to obtain a synchronous signal "e". The synchronous signal "e" is inputted into a video blanking signal generating circuit 3 where it is waveform shaped to obtain the video blanking signal "f". The video blanking signal "f" is assumed to become approximately 0 volts during a blanking period as shown in FIG. 5(b), and to become a fixed voltage Vb during an image period. The signal "f" is inputted into the base of Tr3. The constant current I which becomes EQU I=(Vb-V.sub.BE) / Ro (1)
flows into the collector of Tr3 during the image period. V.sub.BE is the base emitter voltage of the transistor with any transistor being adapted to take approximately the same value. In the image blanking period, the base voltage of Tr3 is approximately 0 volts, so that Tr3 becomes cut off, thus resulting in I=0.
The input video luminance signal "a" is also inputted into the pedestal clamping circuit 1 to obtain the pedestal voltage "b" and the post-clamp video luminance signal "c". Namely, the pedestal level of the signal "c" becomes equal to the pedestal voltage "b". The pedestal voltage "b" and the post-clamp video luminance signal "c" are respectively inputted into the bases of Tr1 and Tr2. Since the transistors form the differential amplifying circuit 5, two relation formulas EQU I=I1=I2 (2) EQU V.sub.B1 -V.sub.B2 =R1.multidot.I1-R8.multidot.I2 (3)
are approximately established, where R1=R8, with the base voltage of Tr1 being V.sub.B1, and the base voltage of Tr2 being V.sub.B2. For example, in a case of APL=0%, the following formula holds, EQU I1=I2=I/2 (4)
The current I2 is inputted into the collector of Tr7 of the current mirror circuit 9 to obtain a corresponding current I5 flowing into the collector of Tr8. Namely, EQU I2=I5 (5)
The current I5 is generated in accordance with the image picture level, is converted into a voltage by R.sub.L and C.sub.L, is simultaneously averaged, and is outputted as the average picture level "d" (FIG. 5f).
Assuming that the output average picture level voltage "d" at this time is V.sub.APL, then, EQU V.sub.APL =V.sub.P +R.sub.L .intg.I5dt (6)
Substituting the formulas (3) and (5) into the formula (6) and, assuming that R1=R8=R, then, EQU V.sub.APL =V.sub.P +R.sub.L /2R.times..intg.(V.sub.B2 -V.sub.B1) dt+R.sub.L .intg.I/2 (7)
It is found that the average value of the image signal is obtained in a second term as intended. The relationship between the input picture level (V.sub.B2 -V.sub.B1) and V.sub.APL is shown in FIG. 6.
In the construction described hereinabove, an offset voltage of R.sub.L .multidot.I/2 is caused in the third term of the formula (7) representing the output average picture level "d". The average picture level voltage in the condition APL=0%, which becomes a reference of the operation of the circuit, changes according to R.sub.L and the current I of the constant current supply as shown in FIG. 6. Also, when an attempt is made to vary the output amplitude by an increase or decrease in the value of R.sub.L, a problem arises in that the output voltage in the condition of APL=0% is also changed at the same time.